US6165387A - Composition for electric cables - Google Patents
Composition for electric cables Download PDFInfo
- Publication number
- US6165387A US6165387A US09/365,939 US36593999A US6165387A US 6165387 A US6165387 A US 6165387A US 36593999 A US36593999 A US 36593999A US 6165387 A US6165387 A US 6165387A
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- acrylate
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- 239000000203 mixture Substances 0.000 title claims abstract description 75
- -1 alkylene glycol Chemical compound 0.000 claims abstract description 23
- 229920001038 ethylene copolymer Polymers 0.000 claims abstract description 23
- LYCAIKOWRPUZTN-UHFFFAOYSA-N ethylene glycol Natural products OCCO LYCAIKOWRPUZTN-UHFFFAOYSA-N 0.000 claims abstract description 23
- WGCNASOHLSPBMP-UHFFFAOYSA-N hydroxyacetaldehyde Natural products OCC=O WGCNASOHLSPBMP-UHFFFAOYSA-N 0.000 claims abstract description 19
- NIXOWILDQLNWCW-UHFFFAOYSA-M Acrylate Chemical compound [O-]C(=O)C=C NIXOWILDQLNWCW-UHFFFAOYSA-M 0.000 claims abstract description 18
- 239000000178 monomer Substances 0.000 claims description 26
- 239000006229 carbon black Substances 0.000 claims description 10
- XTXRWKRVRITETP-UHFFFAOYSA-N Vinyl acetate Chemical compound CC(=O)OC=C XTXRWKRVRITETP-UHFFFAOYSA-N 0.000 claims description 9
- CERQOIWHTDAKMF-UHFFFAOYSA-N Methacrylic acid Chemical compound CC(=C)C(O)=O CERQOIWHTDAKMF-UHFFFAOYSA-N 0.000 claims description 8
- 229910052739 hydrogen Inorganic materials 0.000 claims description 6
- 150000002148 esters Chemical class 0.000 claims description 4
- 239000004711 α-olefin Substances 0.000 claims description 4
- BLRPTPMANUNPDV-UHFFFAOYSA-N Silane Chemical compound [SiH4] BLRPTPMANUNPDV-UHFFFAOYSA-N 0.000 claims description 3
- 229910000077 silane Inorganic materials 0.000 claims description 3
- 229920000642 polymer Polymers 0.000 description 45
- 238000012360 testing method Methods 0.000 description 23
- 240000005572 Syzygium cordatum Species 0.000 description 17
- 235000006650 Syzygium cordatum Nutrition 0.000 description 17
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 16
- VGGSQFUCUMXWEO-UHFFFAOYSA-N Ethene Chemical compound C=C VGGSQFUCUMXWEO-UHFFFAOYSA-N 0.000 description 14
- 239000005977 Ethylene Substances 0.000 description 14
- 239000000463 material Substances 0.000 description 12
- 229920001223 polyethylene glycol Polymers 0.000 description 11
- 239000002202 Polyethylene glycol Substances 0.000 description 10
- 229920001684 low density polyethylene Polymers 0.000 description 10
- 239000004702 low-density polyethylene Substances 0.000 description 10
- 229920001897 terpolymer Polymers 0.000 description 8
- 239000004065 semiconductor Substances 0.000 description 7
- 229940117958 vinyl acetate Drugs 0.000 description 7
- 229920002818 (Hydroxyethyl)methacrylate Polymers 0.000 description 6
- WOBHKFSMXKNTIM-UHFFFAOYSA-N Hydroxyethyl methacrylate Chemical group CC(=C)C(=O)OCCO WOBHKFSMXKNTIM-UHFFFAOYSA-N 0.000 description 6
- 230000032683 aging Effects 0.000 description 6
- 229920001577 copolymer Polymers 0.000 description 6
- 229920000573 polyethylene Polymers 0.000 description 6
- XMNIXWIUMCBBBL-UHFFFAOYSA-N 2-(2-phenylpropan-2-ylperoxy)propan-2-ylbenzene Chemical compound C=1C=CC=CC=1C(C)(C)OOC(C)(C)C1=CC=CC=C1 XMNIXWIUMCBBBL-UHFFFAOYSA-N 0.000 description 5
- 239000005038 ethylene vinyl acetate Substances 0.000 description 5
- 230000002401 inhibitory effect Effects 0.000 description 5
- 229920001200 poly(ethylene-vinyl acetate) Polymers 0.000 description 5
- 150000003254 radicals Chemical class 0.000 description 5
- UEQXEQXNHPQBQO-UHFFFAOYSA-N 2-[2-[2-[2-[2-(2-hydroxyethoxy)ethoxy]ethoxy]ethoxy]ethoxy]ethyl 2-methylprop-2-enoate Chemical compound CC(=C)C(=O)OCCOCCOCCOCCOCCOCCO UEQXEQXNHPQBQO-UHFFFAOYSA-N 0.000 description 4
- BAPJBEWLBFYGME-UHFFFAOYSA-N Methyl acrylate Chemical compound COC(=O)C=C BAPJBEWLBFYGME-UHFFFAOYSA-N 0.000 description 4
- 230000000052 comparative effect Effects 0.000 description 4
- 239000003999 initiator Substances 0.000 description 4
- VXNZUUAINFGPBY-UHFFFAOYSA-N 1-Butene Chemical compound CCC=C VXNZUUAINFGPBY-UHFFFAOYSA-N 0.000 description 3
- SMZOUWXMTYCWNB-UHFFFAOYSA-N 2-(2-methoxy-5-methylphenyl)ethanamine Chemical compound COC1=CC=C(C)C=C1CCN SMZOUWXMTYCWNB-UHFFFAOYSA-N 0.000 description 3
- DNIAPMSPPWPWGF-UHFFFAOYSA-N Propylene glycol Chemical compound CC(O)CO DNIAPMSPPWPWGF-UHFFFAOYSA-N 0.000 description 3
- NIXOWILDQLNWCW-UHFFFAOYSA-N acrylic acid group Chemical group C(C=C)(=O)O NIXOWILDQLNWCW-UHFFFAOYSA-N 0.000 description 3
- 239000004020 conductor Substances 0.000 description 3
- 239000011810 insulating material Substances 0.000 description 3
- 150000002978 peroxides Chemical class 0.000 description 3
- AZIQALWHRUQPHV-UHFFFAOYSA-N prop-2-eneperoxoic acid Chemical compound OOC(=O)C=C AZIQALWHRUQPHV-UHFFFAOYSA-N 0.000 description 3
- KWKAKUADMBZCLK-UHFFFAOYSA-N 1-octene Chemical compound CCCCCCC=C KWKAKUADMBZCLK-UHFFFAOYSA-N 0.000 description 2
- 239000004698 Polyethylene Substances 0.000 description 2
- QQONPFPTGQHPMA-UHFFFAOYSA-N Propene Chemical compound CC=C QQONPFPTGQHPMA-UHFFFAOYSA-N 0.000 description 2
- 239000000654 additive Substances 0.000 description 2
- 230000008901 benefit Effects 0.000 description 2
- 230000015572 biosynthetic process Effects 0.000 description 2
- 239000003795 chemical substances by application Substances 0.000 description 2
- 238000005336 cracking Methods 0.000 description 2
- 230000006353 environmental stress Effects 0.000 description 2
- 230000014509 gene expression Effects 0.000 description 2
- 125000002887 hydroxy group Chemical group [H]O* 0.000 description 2
- 238000004519 manufacturing process Methods 0.000 description 2
- 238000000034 method Methods 0.000 description 2
- 238000002156 mixing Methods 0.000 description 2
- 125000003903 2-propenyl group Chemical group [H]C([*])([H])C([H])=C([H])[H] 0.000 description 1
- GNSFRPWPOGYVLO-UHFFFAOYSA-N 3-hydroxypropyl 2-methylprop-2-enoate Chemical compound CC(=C)C(=O)OCCCO GNSFRPWPOGYVLO-UHFFFAOYSA-N 0.000 description 1
- QZPSOSOOLFHYRR-UHFFFAOYSA-N 3-hydroxypropyl prop-2-enoate Chemical compound OCCCOC(=O)C=C QZPSOSOOLFHYRR-UHFFFAOYSA-N 0.000 description 1
- TVAAPIVBJRKZHE-UHFFFAOYSA-N C(C(=C)C)(=O)OCCO.C(C)(=O)OC=C.C=C Chemical compound C(C(=C)C)(=O)OCCO.C(C)(=O)OC=C.C=C TVAAPIVBJRKZHE-UHFFFAOYSA-N 0.000 description 1
- RYGMFSIKBFXOCR-UHFFFAOYSA-N Copper Chemical compound [Cu] RYGMFSIKBFXOCR-UHFFFAOYSA-N 0.000 description 1
- MHAJPDPJQMAIIY-UHFFFAOYSA-N Hydrogen peroxide Chemical compound OO MHAJPDPJQMAIIY-UHFFFAOYSA-N 0.000 description 1
- CERQOIWHTDAKMF-UHFFFAOYSA-M Methacrylate Chemical compound CC(=C)C([O-])=O CERQOIWHTDAKMF-UHFFFAOYSA-M 0.000 description 1
- 101100386054 Saccharomyces cerevisiae (strain ATCC 204508 / S288c) CYS3 gene Proteins 0.000 description 1
- 239000002253 acid Substances 0.000 description 1
- 230000009471 action Effects 0.000 description 1
- 239000000853 adhesive Substances 0.000 description 1
- 230000001070 adhesive effect Effects 0.000 description 1
- 125000003545 alkoxy group Chemical group 0.000 description 1
- 125000005907 alkyl ester group Chemical group 0.000 description 1
- 125000002947 alkylene group Chemical group 0.000 description 1
- QVGXLLKOCUKJST-UHFFFAOYSA-N atomic oxygen Chemical compound [O] QVGXLLKOCUKJST-UHFFFAOYSA-N 0.000 description 1
- 230000004888 barrier function Effects 0.000 description 1
- 229920005601 base polymer Polymers 0.000 description 1
- IAQRGUVFOMOMEM-UHFFFAOYSA-N butene Natural products CC=CC IAQRGUVFOMOMEM-UHFFFAOYSA-N 0.000 description 1
- 125000000484 butyl group Chemical group [H]C([*])([H])C([H])([H])C([H])([H])C([H])([H])[H] 0.000 description 1
- QYMGIIIPAFAFRX-UHFFFAOYSA-N butyl prop-2-enoate;ethene Chemical compound C=C.CCCCOC(=O)C=C QYMGIIIPAFAFRX-UHFFFAOYSA-N 0.000 description 1
- 125000004432 carbon atom Chemical group C* 0.000 description 1
- 125000002915 carbonyl group Chemical group [*:2]C([*:1])=O 0.000 description 1
- 125000003178 carboxy group Chemical group [H]OC(*)=O 0.000 description 1
- 230000015556 catabolic process Effects 0.000 description 1
- 238000006243 chemical reaction Methods 0.000 description 1
- 230000003749 cleanliness Effects 0.000 description 1
- 239000011248 coating agent Substances 0.000 description 1
- 150000001875 compounds Chemical class 0.000 description 1
- 238000010276 construction Methods 0.000 description 1
- 229910052802 copper Inorganic materials 0.000 description 1
- 239000010949 copper Substances 0.000 description 1
- 229920003020 cross-linked polyethylene Polymers 0.000 description 1
- 239000004703 cross-linked polyethylene Substances 0.000 description 1
- 238000000354 decomposition reaction Methods 0.000 description 1
- 230000007547 defect Effects 0.000 description 1
- 238000010586 diagram Methods 0.000 description 1
- 239000003989 dielectric material Substances 0.000 description 1
- 230000000694 effects Effects 0.000 description 1
- 230000005684 electric field Effects 0.000 description 1
- 125000004185 ester group Chemical group 0.000 description 1
- NKSJNEHGWDZZQF-UHFFFAOYSA-N ethenyl(trimethoxy)silane Chemical compound CO[Si](OC)(OC)C=C NKSJNEHGWDZZQF-UHFFFAOYSA-N 0.000 description 1
- 229920006245 ethylene-butyl acrylate Polymers 0.000 description 1
- 235000012438 extruded product Nutrition 0.000 description 1
- 238000001125 extrusion Methods 0.000 description 1
- 230000002349 favourable effect Effects 0.000 description 1
- 125000000524 functional group Chemical group 0.000 description 1
- 229920001519 homopolymer Polymers 0.000 description 1
- 239000012943 hotmelt Substances 0.000 description 1
- 239000001257 hydrogen Substances 0.000 description 1
- UACSZOWTRIJIFU-UHFFFAOYSA-N hydroxymethyl 2-methylprop-2-enoate Chemical compound CC(=C)C(=O)OCO UACSZOWTRIJIFU-UHFFFAOYSA-N 0.000 description 1
- 239000012535 impurity Substances 0.000 description 1
- 238000007373 indentation Methods 0.000 description 1
- 239000000976 ink Substances 0.000 description 1
- 239000000155 melt Substances 0.000 description 1
- 239000002184 metal Substances 0.000 description 1
- 229910052751 metal Inorganic materials 0.000 description 1
- 125000005395 methacrylic acid group Chemical group 0.000 description 1
- VLKZOEOYAKHREP-UHFFFAOYSA-N n-Hexane Chemical compound CCCCCC VLKZOEOYAKHREP-UHFFFAOYSA-N 0.000 description 1
- TVMXDCGIABBOFY-UHFFFAOYSA-N n-Octanol Natural products CCCCCCCC TVMXDCGIABBOFY-UHFFFAOYSA-N 0.000 description 1
- 229910052760 oxygen Inorganic materials 0.000 description 1
- 239000001301 oxygen Substances 0.000 description 1
- 239000003973 paint Substances 0.000 description 1
- 229920003023 plastic Polymers 0.000 description 1
- 239000004033 plastic Substances 0.000 description 1
- 229920001515 polyalkylene glycol Polymers 0.000 description 1
- 229920000151 polyglycol Polymers 0.000 description 1
- 239000010695 polyglycol Substances 0.000 description 1
- 239000002861 polymer material Substances 0.000 description 1
- 229920000098 polyolefin Polymers 0.000 description 1
- 230000008569 process Effects 0.000 description 1
- 239000003381 stabilizer Substances 0.000 description 1
- 101150035983 str1 gene Proteins 0.000 description 1
- 230000035900 sweating Effects 0.000 description 1
- 229920001567 vinyl ester resin Polymers 0.000 description 1
- 125000000391 vinyl group Chemical group [H]C([*])=C([H])[H] 0.000 description 1
- 210000002268 wool Anatomy 0.000 description 1
Classifications
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01B—CABLES; CONDUCTORS; INSULATORS; SELECTION OF MATERIALS FOR THEIR CONDUCTIVE, INSULATING OR DIELECTRIC PROPERTIES
- H01B7/00—Insulated conductors or cables characterised by their form
- H01B7/17—Protection against damage caused by external factors, e.g. sheaths or armouring
- H01B7/28—Protection against damage caused by moisture, corrosion, chemical attack or weather
- H01B7/2813—Protection against damage caused by electrical, chemical or water tree deterioration
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01B—CABLES; CONDUCTORS; INSULATORS; SELECTION OF MATERIALS FOR THEIR CONDUCTIVE, INSULATING OR DIELECTRIC PROPERTIES
- H01B1/00—Conductors or conductive bodies characterised by the conductive materials; Selection of materials as conductors
- H01B1/20—Conductive material dispersed in non-conductive organic material
- H01B1/24—Conductive material dispersed in non-conductive organic material the conductive material comprising carbon-silicon compounds, carbon or silicon
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01B—CABLES; CONDUCTORS; INSULATORS; SELECTION OF MATERIALS FOR THEIR CONDUCTIVE, INSULATING OR DIELECTRIC PROPERTIES
- H01B3/00—Insulators or insulating bodies characterised by the insulating materials; Selection of materials for their insulating or dielectric properties
- H01B3/18—Insulators or insulating bodies characterised by the insulating materials; Selection of materials for their insulating or dielectric properties mainly consisting of organic substances
- H01B3/30—Insulators or insulating bodies characterised by the insulating materials; Selection of materials for their insulating or dielectric properties mainly consisting of organic substances plastics; resins; waxes
- H01B3/44—Insulators or insulating bodies characterised by the insulating materials; Selection of materials for their insulating or dielectric properties mainly consisting of organic substances plastics; resins; waxes vinyl resins; acrylic resins
- H01B3/441—Insulators or insulating bodies characterised by the insulating materials; Selection of materials for their insulating or dielectric properties mainly consisting of organic substances plastics; resins; waxes vinyl resins; acrylic resins from alkenes
Definitions
- the present invention relates to a composition for electric cables. More specifically, the invention relates to a (meth)acrylate-ester-containing ethylene polymer which is usable in compositions for electric cables, more particularly in compositions for inner and outer semiconducting layers as well as insulating layers for electric cables.
- Electric cables and particularly electric power cables for medium and high voltages are composed of a plurality of polymer layers extruded round the electric conductor.
- the electric conductor is usually coated first with an inner semiconductor layer followed by an insulating layer, then an outer semiconductor layer followed by water barrier layers, if any, and on the outside a sheath layer.
- the insulating layer and the semiconductor layers normally consist of cross-linked ethylene homo- and/or copolymers.
- LDPE low density polyethylene, i.e. polyethylene prepared by radical polymerisation at a high pressure
- peroxide for instance dicumyl peroxide
- a limitation of conventional LDPE is its tendency to be exposed, in the presence of water and under the action of strong electric fields, to the formation of dendritically branched defects, so-called water trees, which can lead to breakdown and possible electric failure. This tendency is strongly affected by the presence of inhomogeneities, microcavities and impurities in the material.
- EP-A-0 057 604 it is known to inhibit water treeing by adding to a semi-conducting composition, which mainly consists of a polyolefin and 5-50% by weight carbon black based on the weight of the total composition, a polyethylene glycol having a molecular weight of about 1000-20000 in an amount of 0.1-20% by weight.
- This composition is intended for semiconducting layers of electric cables and by adding polyethylene glycol, it is said to be possible to eliminate water trees which grow into the insulating layer from the interface between the insulating layer and the semiconducting layer.
- U.S. Patent Specification U.S. Pat. No. 4,812,505 discloses a composition, which is usable as insulating layer in electric cables and which is resistant to water treeing.
- the composition comprises a copolymer of ethylene and at least one alpha-olefin having 4-8 carbon atoms, such as 1-butene, 1-hexane or 1-octene, and besides comprises a polyethylene glycol having a molecular weight in the range of about 1000-20000 in an amount of 0.1-20% by weight.
- EP-A-0 538 033 discloses an extrudable ethylene hydroxy acrylate co- or terpolymer, which, in addition to ethylene, comprises 7-30% by weight hydroxy acrylate, and 0-40% by weight of a third monomer selected among vinyl esters, allyl esters, and acrylic or methacrylic esters which do not contain hydroxyl groups.
- the hydroxy acrylate may consist of any ester of glycol or polyglycol and acrylic acid or methacrylic acid, but preferably is hydroxyethyl methacrylate, hydroxymethyl methacrylate, hydroxypropyl acrylate or hydroxypropyl methacrylate.
- hydroxy acrylate is previously known in connection with hot melts, and the extruded product according to the patent specification, e.g. a film, is hydrophilic and absorbs and transmits moisture, the ethylene hydroxy acrylate copolymer improving the adhesion to, for instance, polar plastics and other materials and improves the strength properties because of the hydrogen bonds.
- the patent specification does not indicate the use of the polymer in compositions for electric cables.
- an ethylene copolymer which comprises 25-99.9% by weight ethylene, 75-0.1% by weight polyalkylene glycol monoacrylate, and 0-65% by weight of other ethylenically unsaturated monomers.
- This polymer is said to be usable for paints, inks etc, as coating agents for metal, paper, wool etc, as adhesives etc.
- the use of the polymer in compositions for electric cables is not disclosed.
- n 1-20.
- composition according to the invention in insulating and semi-conducting layers for electric cables.
- ethylene copolymer here used is meant an ethylene-based polymer, which is obtained by polymerisation of ethylene and one or more other monomers, one of these other monomers consisting of (poly)alkylene glycol mono(meth)acrylate having the formula I.
- the ethylene copolymer consists of a polymer of ethylene and the monomer of formula I, or of ethylene, the monomer of formula I and a further monomer, i.e. in the latter case a so-called terpolymer.
- (meth)acrylic acid and “(meth)acrylate” here used are meant acrylic acid and acrylate as well as methacrylic acid and methacrylate.
- R 1 in formula I is CH 3 , i.e. the esterifying acid is methacrylic acid, and R 2 in formula I is H, i.e. the esterifying (poly)alkylene glycol is a (poly)ethylene glycol.
- the monomer of formula I is hydroxyethyl methacrylate (HEMA) with the stated, preferred meanings of R 1 and R 2 .
- HEMA hydroxyethyl methacrylate
- R 1 and R 2 hexaethylene glycol methacrylate
- the amount of the comonomer of formula I in the ethylene copolymer may vary within wide limits, but preferably constitutes about 0.1-15% by weight, more preferred about 2-14% by weight of the copolymer.
- the ethylene copolymer may optionally comprise further comonomers in addition to the comonomer of formula I, and it is preferred that the comonomer contains such a further monomer, i.e. that the copolymer is a terpolymer.
- This further comonomer can be selected among monomers, which are copolymerisable with ethylene and (poly)alkylene glycol mono(meth)acrylate of formula I.
- Such monomers are well known to those skilled in the art and no extensive enumeration will be required, but as examples, mention can be made of vinylically unsaturated monomers, such as C 3 -C 8 alpha olefins, for instance propene, butene etc.; vinylically unsaturated monomers containing functional groups, such as hydroxyl groups, alkoxy groups, carbonyl groups, carboxyl groups and ester groups.
- Such monomers may consist of, for instance, (meth)acrylic acid and alkyl esters thereof, such as methyl-, ethyl- and butyl(meth)acrylate; vinylically unsaturated, hydrolysable silane monomers, such as vinyl trimethoxysilane; vinyl acetate etc.
- the amount of further comonomer(s) in addition to (poly)alkylene glycol mono(meth)acrylate of formula I is from 0 to about 40% by weight, preferably about 1-30% by weight of the ethylene copolymer.
- the sum of all the monomer contents is 100% by weight.
- Graft copolymerisation is a per se well-known polymerisation process in the art and therefore, no detailed description will be needed.
- graft copolymerisation is carried out by copolymerisation of a vinylically unsaturated monomer with an ethylene polymer, such as an ethylene homopolymer or an ethylene copolymer, under the influence of a free radical initiator, such as a peroxide, for instance dicumyl peroxide (DCP).
- DCP dicumyl peroxide
- the temperature in the graft copolymerisation should be sufficient for decomposition of the free radical initiator with formation of free radicals, which, by using dicumyl peroxide as initiator, means about 150-200° C., and the polymerisation can be practically effected, for instance, by mixing the components in an extruder.
- the inventive ethylene copolymer also leads to other favourable and desirable properties when using it as material in electric cables.
- the ethylene copolymer according to the invention permits an improved dielectric strength, which is of value both to the insulating layer and the semiconducting layers of an electric cable.
- the inventive ethylene copolymer has a good environmental stress cracking resistance (ESCR), which is of value for semiconducting layers of electric cables.
- the water tree resistance (WTR) was determined for three polymer compositions, Polymer 1, Polymer 2 and Polymer 3, by so-called AshcraEt testing.
- Ashcraft testing which is a testing method for determining the WTR properties of polymers, has been described by Ashcraft, A. C., "Water Treeing in Polymeric Dielectrics", World Electrotechnical Congress in Moscow, USSR, Jun. 22, 1977.
- Ashcraft testing well characterised effects are provided, viz. sharp, water-filled indentations, by means of a needle in compression-moulded cups.
- a voltage of 5 kV/6 kHZ is applied across the water, whereas the bottom of the cup is connected to earth.
- the temperature is constantly kept at 65° C.
- the average length of the water trees after 72 h ageing is considered as a measure of the growth rate of the water trees in the specific insulating material.
- Polymer 1 consisted of a low-density polyethylene (LDPE) having a melt flow rate (MFR) of 2 g/10 min, which was used as reference
- Polymer 2 consisted of 99.1 parts by weight of the same type of LDPE, to which 0.56 parts by weight of polyethylene glycol (PEG) having a molecular weight of about 20000 had been added as a conventional agent inhibiting water treeing
- the polymer compositions also contained about 2 parts by weight of dicumyl peroxide as well as
- the water tree resistance of two compositions according to the invention was compared by Ashcraft testing.
- LDPE low-density polyethylene
- HEMA hydroxyethyl methacrylate
- One composition contained 6.5% by weight of the water-tree-inhibiting polymer, whereas the other composition contained 14% by weight thereof.
- the second composition (Polymer B) consisted of the same EVA polymer as in the first composition, with the difference that 0.6% by weight polyethylene glycol (PEG) having a molecular weight of about 20000 had been added. Besides, the composition contained about 40% by weight carbon black. This composition was an example of prior art.
- the third composition (Polymer C) consisted of a terpolymer of ethylene, 18% by weight vinyl acetate and 3% by weight of a monomer of formula I.
- the composition contained about 40% by weight carbon black. This composition was a composition according to the invention.
- Each of the three compositions above was incorporated as inner semiconducting layers in electric cables, which, seen from inside and outwards, consisted of a 1.4 mm copper conductor, an inner semiconducting layer having an outer diameter of 2.8 mm, an insulating layer having an outer diameter of 5.8 mm and an outer semiconducting ducting layer having an outer diameter of 6.1 mm.
- the insulating layer consisted of low-density polyethylene having an MFR of 2 g/10 min
- the outer semiconducting layer consisted of an ethylene butyl acrylate copolymer with an addition of about 40% by weight carbon black.
- the testing of the dielectric strength was carried out on these test cables in accordance with a method developed by Alcatel AG & Co, Hannover, Germany, and described in an article by Land H. G., Schadlich Hans, "Model Cable Test for Evaluating the Ageing Behaviour under Water Influence of Compounds for Medium Voltage Cables", Conference Proceedings of Jlcable 91, 24-28 June 1991, Versaille, France.
- As a value of the dielectric strength is stated 63% of E max from Weibull diagram in kV/mm.
- the dielectric strength was measured on the one hand A) after ageing for 16 h at 90° C. in air and, on the other hand, B) after ageing for 1000 h at 9 kV/mm in 85/70° C. water. The results of the testing are stated in the Table below.
- the composition according to the invention showed good properties as inner semiconducting layers and, in particular, had an excellent electric dielectric strength after ageing for 1000 h at 9 kV/mm in 85/70° C. water.
- Example 3 the dielectric strength was tested in a similar fashion as in Example 3 on an electric cable, which had as inner semiconducting layer a composition consisting of a terpolymer of ethylene, about 15% by weight methylacrylate and about 2% by weight hexaethylene glycol monomethacrylate, i.e. the same monomer of formula I as in Example 3, and about 40% by weight carbon black.
- a dielectric strength (63% of E max ) of 59.4 kV/mm was obtained after ageing for 1000 h at 9 kV/mm in 85/70° C. water.
- the environmental stress cracking resistance was tested, a property that is important, particularly to the outer semiconducting layer of an electric cable.
- the testing was carried out according to ASTM D 1693, on the one hand with 10% Igepal at 50° C. and, on the other hand, in air at 50° C.
- the result of the testing of ESCR is shown in the Table below and is stated as the number of test pieces of a total of 10 test pieces that were broken in the testing after a certain time, measured in hours.
- the composition according to the invention had a considerably improved ESCR and is consequently well suited as material for outer semiconducting layers of electric cables.
Abstract
A composition for electric cables is described. The composition comprises an ethylene copolymer which includes a (poly)alkylene glycol mono(meth)acrylate of formula (I) wherein R1 =H or CH3 ; R2 =H or CH3 ; n=1-20. The composition may be used as an insulating layer or a semiconducting layer of an electric cable.
Description
This application is a continuation of international application number PCT/SE98/00013, filed Jan. 9, 1998, pending.
The present invention relates to a composition for electric cables. More specifically, the invention relates to a (meth)acrylate-ester-containing ethylene polymer which is usable in compositions for electric cables, more particularly in compositions for inner and outer semiconducting layers as well as insulating layers for electric cables.
Electric cables and particularly electric power cables for medium and high voltages are composed of a plurality of polymer layers extruded round the electric conductor. The electric conductor is usually coated first with an inner semiconductor layer followed by an insulating layer, then an outer semiconductor layer followed by water barrier layers, if any, and on the outside a sheath layer.
The insulating layer and the semiconductor layers normally consist of cross-linked ethylene homo- and/or copolymers. LDPE (low density polyethylene, i.e. polyethylene prepared by radical polymerisation at a high pressure) cross-linked by adding peroxide, for instance dicumyl peroxide, in connection with the extrusion of the cable, is today the predominant cable insulating material. A limitation of conventional LDPE is its tendency to be exposed, in the presence of water and under the action of strong electric fields, to the formation of dendritically branched defects, so-called water trees, which can lead to breakdown and possible electric failure. This tendency is strongly affected by the presence of inhomogeneities, microcavities and impurities in the material. Water treeing has been studied carefully, especially since the 1970's, when polymer materials and, in particular, cross-linked polyethylene became the predominant insulating material for electric cables for medium and high voltages. In the past years, these studies have entailed improvements in the construction of the cables, the manufacturing procedure and the quality and cleanliness of the used materials. These improvements have resulted in an increased service life of the manufactured cables. Yet there is a pronounced need of still more improved materials in respect of resistance and water treeing. Such an improved resistance to water treeing is desirable not only for insulating layer materials, but also for semiconductor layer materials of electric cables. Another important property of semiconductor layer materials of electric cables is high resistance to form cracks.
From European Patent Specification EP-A-0 057 604 it is known to inhibit water treeing by adding to a semi-conducting composition, which mainly consists of a polyolefin and 5-50% by weight carbon black based on the weight of the total composition, a polyethylene glycol having a molecular weight of about 1000-20000 in an amount of 0.1-20% by weight. This composition is intended for semiconducting layers of electric cables and by adding polyethylene glycol, it is said to be possible to eliminate water trees which grow into the insulating layer from the interface between the insulating layer and the semiconducting layer.
Moreover, U.S. Patent Specification U.S. Pat. No. 4,812,505 discloses a composition, which is usable as insulating layer in electric cables and which is resistant to water treeing. The composition comprises a copolymer of ethylene and at least one alpha-olefin having 4-8 carbon atoms, such as 1-butene, 1-hexane or 1-octene, and besides comprises a polyethylene glycol having a molecular weight in the range of about 1000-20000 in an amount of 0.1-20% by weight.
The drawback of using water-tree-inhibiting additives, such as polyethylene glycol, is that there is a risk, owing to the insufficient compatibility of the polyethylene glycol with the base polymer (polyethylene), of sweating out the polyethylene glycol, especially if its molecular weight is not high. On the other hand, if the molecular weight is high, the possibility of efficient mixing is affected negatively.
European Patent Specification EP-A-0 538 033 discloses an extrudable ethylene hydroxy acrylate co- or terpolymer, which, in addition to ethylene, comprises 7-30% by weight hydroxy acrylate, and 0-40% by weight of a third monomer selected among vinyl esters, allyl esters, and acrylic or methacrylic esters which do not contain hydroxyl groups. The hydroxy acrylate may consist of any ester of glycol or polyglycol and acrylic acid or methacrylic acid, but preferably is hydroxyethyl methacrylate, hydroxymethyl methacrylate, hydroxypropyl acrylate or hydroxypropyl methacrylate. According to the patent specification, hydroxy acrylate is previously known in connection with hot melts, and the extruded product according to the patent specification, e.g. a film, is hydrophilic and absorbs and transmits moisture, the ethylene hydroxy acrylate copolymer improving the adhesion to, for instance, polar plastics and other materials and improves the strength properties because of the hydrogen bonds. The patent specification does not indicate the use of the polymer in compositions for electric cables.
From Derwent's Abstract No. 77-85827Y/48 of Japanese Patent Application JP 7644050, an ethylene copolymer is known, which comprises 25-99.9% by weight ethylene, 75-0.1% by weight polyalkylene glycol monoacrylate, and 0-65% by weight of other ethylenically unsaturated monomers. This polymer is said to be usable for paints, inks etc, as coating agents for metal, paper, wool etc, as adhesives etc. The use of the polymer in compositions for electric cables is not disclosed.
According to the present invention, it has now surprisingly been found that the use of (poly)alkylene glycol mono(meth)acrylate as comonomer in ethylene polymers renders it possible to provide compositions for electric cables having improved resistance to water treeing.
According to the invention, a composition for electric cables thus is provided, characterised in it comprises an ethylene copolymer which includes as a comonomer a (poly)alkylene glycol mono(meth)acrylate having the formula I ##STR1## wherein R1 =H or CH3, R2 H or CH3,
n=1-20.
More particularly, it is contemplated to use the composition according to the invention in insulating and semi-conducting layers for electric cables.
Other distinguishing features and advantages of the invention will appear from the following specification and the appended claims.
By the expression "ethylene copolymer" here used is meant an ethylene-based polymer, which is obtained by polymerisation of ethylene and one or more other monomers, one of these other monomers consisting of (poly)alkylene glycol mono(meth)acrylate having the formula I. Preferably, the ethylene copolymer consists of a polymer of ethylene and the monomer of formula I, or of ethylene, the monomer of formula I and a further monomer, i.e. in the latter case a so-called terpolymer.
By the expressions "(meth)acrylic acid" and "(meth)acrylate" here used are meant acrylic acid and acrylate as well as methacrylic acid and methacrylate.
As appears from formula I above, the (poly)alkylene glycol mono(meth)acrylate of formula I is an ester of acrylic or methacrylic acid with a (poly)alkylene glycol, the alkylene glycol being selected among ethylene glycol or propylene glycol, and the number of alkylene oxide units being variable from 1 to 20, i.e. n=1-20, preferably 1-10 in formula l. Preferably R1 in formula I is CH3, i.e. the esterifying acid is methacrylic acid, and R2 in formula I is H, i.e. the esterifying (poly)alkylene glycol is a (poly)ethylene glycol. When n=1 the monomer of formula I is hydroxyethyl methacrylate (HEMA) with the stated, preferred meanings of R1 and R2. When n=6 the monomer of formula I is hexaethylene glycol methacrylate with the stated, preferred meanings of R1 and R2.
The amount of the comonomer of formula I in the ethylene copolymer may vary within wide limits, but preferably constitutes about 0.1-15% by weight, more preferred about 2-14% by weight of the copolymer.
As stated above, the ethylene copolymer may optionally comprise further comonomers in addition to the comonomer of formula I, and it is preferred that the comonomer contains such a further monomer, i.e. that the copolymer is a terpolymer. This further comonomer can be selected among monomers, which are copolymerisable with ethylene and (poly)alkylene glycol mono(meth)acrylate of formula I. Such monomers are well known to those skilled in the art and no extensive enumeration will be required, but as examples, mention can be made of vinylically unsaturated monomers, such as C3 -C8 alpha olefins, for instance propene, butene etc.; vinylically unsaturated monomers containing functional groups, such as hydroxyl groups, alkoxy groups, carbonyl groups, carboxyl groups and ester groups. Such monomers may consist of, for instance, (meth)acrylic acid and alkyl esters thereof, such as methyl-, ethyl- and butyl(meth)acrylate; vinylically unsaturated, hydrolysable silane monomers, such as vinyl trimethoxysilane; vinyl acetate etc.
The amount of further comonomer(s) in addition to (poly)alkylene glycol mono(meth)acrylate of formula I is from 0 to about 40% by weight, preferably about 1-30% by weight of the ethylene copolymer.
Regarding the above-mentioned monomers, the sum of all the monomer contents is 100% by weight.
The inventive ethylene copolymer can be prepared by graft copolymerisation or by free-radical-initiated high-pressure polymerisation.
Graft copolymerisation is a per se well-known polymerisation process in the art and therefore, no detailed description will be needed. In general terms, graft copolymerisation is carried out by copolymerisation of a vinylically unsaturated monomer with an ethylene polymer, such as an ethylene homopolymer or an ethylene copolymer, under the influence of a free radical initiator, such as a peroxide, for instance dicumyl peroxide (DCP). The temperature in the graft copolymerisation should be sufficient for decomposition of the free radical initiator with formation of free radicals, which, by using dicumyl peroxide as initiator, means about 150-200° C., and the polymerisation can be practically effected, for instance, by mixing the components in an extruder.
Free-radical-initiated high-pressure polymerisation, which is also well known in the art, is generally carried out by reacting, in a reactor, such as an autoclave or tube reactor, at a high pressure of about 100-300 MPa and an elevated temperature of about 80-300° C., the monomers under the influence of a radical initiator, such as a peroxide, hydroperoxide, oxygen or azo compound. When the reaction is completed, the temperature and the pressure are lowered, and the resultant unsaturated polymer is recovered. For further details regarding the production of ethylene polymers by high-pressure polymerisation during free-radical-initiation, reference can be made to Encyclopedia of Polymer Science and Engineering, Volume 6 (1986), pp 383-410, especially pp 404-407.
As mentioned above, it has been found according to the present invention that use of (poly)alkylene glycol mono(meth)acrylate of formula I in an ethylene copolymer results in enhanced water tree resistance (WTR), and therefore such an ethylene copolymer is usable as material for electric cables, for instance as insulating layer material or as semiconductor layer material. By (poly)alkylene glycol mono(meth)acrylate of formula I, which produces water tree resistance, being polymerised into the polymer, it is fixedly anchored in the polymer molecule and cannot migrate or be sweated out, as is the case with conventional WTR additives. This is a special advantage of the inventive polymers. In addition to the advantageous water tree resistance, it has been found that the inventive ethylene copolymer also leads to other favourable and desirable properties when using it as material in electric cables. Thus, it has been found that the ethylene copolymer according to the invention permits an improved dielectric strength, which is of value both to the insulating layer and the semiconducting layers of an electric cable. Further, the inventive ethylene copolymer has a good environmental stress cracking resistance (ESCR), which is of value for semiconducting layers of electric cables.
To further facilitate the understanding of the invention, some elucidative, but not restrictive Examples and comparative Examples will be given below.
The water tree resistance (WTR) was determined for three polymer compositions, Polymer 1, Polymer 2 and Polymer 3, by so-called AshcraEt testing.
Ashcraft testing, which is a testing method for determining the WTR properties of polymers, has been described by Ashcraft, A. C., "Water Treeing in Polymeric Dielectrics", World Electrotechnical Congress in Moscow, USSR, Jun. 22, 1977. By Ashcraft testing, well characterised effects are provided, viz. sharp, water-filled indentations, by means of a needle in compression-moulded cups. A voltage of 5 kV/6 kHZ is applied across the water, whereas the bottom of the cup is connected to earth. The temperature is constantly kept at 65° C. The average length of the water trees after 72 h ageing is considered as a measure of the growth rate of the water trees in the specific insulating material.
For the testing, compression-moulded test pieces were prepared from the various polymers, of which Polymer 1 consisted of a low-density polyethylene (LDPE) having a melt flow rate (MFR) of 2 g/10 min, which was used as reference, Polymer 2 consisted of 99.1 parts by weight of the same type of LDPE, to which 0.56 parts by weight of polyethylene glycol (PEG) having a molecular weight of about 20000 had been added as a conventional agent inhibiting water treeing, and Polymer 3, which was a composition according to the invention, consisted of 79.8 parts by weight of the same type of LDPE, to which had been added 20.0 parts by weight of a terpolymer of ethylene, methylacrylate (13% by weight), and hexaethylene glycol monomethacrylate (3% by weight) of formula I, wherein R1 =CH3, R2 =H, and n=6. The polymer compositions also contained about 2 parts by weight of dicumyl peroxide as well as a stabiliser (about 0.2 parts by weight). The results from the Ashcraft testing are compiled in the Table below.
______________________________________ Water tree Average length Average length Composition (μm) (%) ______________________________________ Polymer 1 (reference) 374 100 Polymer 2 (comparative) 149 40 Polymer 3 (inventive) 126 34 ______________________________________
The test results clearly show the enhanced WTR properties of the composition according to the invention.
In this Example, the water tree resistance of two compositions according to the invention was compared by Ashcraft testing. The compositions consisted of low-density polyethylene (LDPE) having an MER=2 g/10 min in combination with different contents of a water-tree-inhibiting polymer, which consisted of a terpolymer of ethylene, 20% by weight vinyl acetate, and 9% by weight hydroxyethyl methacrylate (HEMA which is a comonomer of formula I, wherein R1 =CH3, R2 =H and n=1). One composition contained 6.5% by weight of the water-tree-inhibiting polymer, whereas the other composition contained 14% by weight thereof. By Ashcraft testing, an average length of the water trees, counted in % of the average length of the water trees for the reference polymer in Example 1, of 46% for the composition having 6.5% by weight EVA-HEMA and 21% for the composition having 14% by weight EVA-HEMA. Thus, it is evident that the water tree resistance increases with an increasing content of the water-tree-inhibiting polymer containing the monomer of formula I.
In this Example, the dielectric strength of three semiconducting polymer compositions was measured, viz. polymer A, B and C, which constituted the inner semiconductor of an electric cable.
The first composition (Polymer A) consisted of an ethylene vinyl acetate copolymer (EVA) having 18% by weight vinylacetate, the composition containing about 40% by weight carbon black in order to make the composition semiconducting. This composition was used as reference.
The second composition (Polymer B) consisted of the same EVA polymer as in the first composition, with the difference that 0.6% by weight polyethylene glycol (PEG) having a molecular weight of about 20000 had been added. Besides, the composition contained about 40% by weight carbon black. This composition was an example of prior art.
The third composition (Polymer C) consisted of a terpolymer of ethylene, 18% by weight vinyl acetate and 3% by weight of a monomer of formula I. The monomer of formula I consisted of hexaethylene glycol monomethacrylate, i.e. R1 =CH3, R2 =H and n=6 in formula I. Moreover, the composition contained about 40% by weight carbon black. This composition was a composition according to the invention.
Each of the three compositions above was incorporated as inner semiconducting layers in electric cables, which, seen from inside and outwards, consisted of a 1.4 mm copper conductor, an inner semiconducting layer having an outer diameter of 2.8 mm, an insulating layer having an outer diameter of 5.8 mm and an outer semiconducting ducting layer having an outer diameter of 6.1 mm. The insulating layer consisted of low-density polyethylene having an MFR of 2 g/10 min, and the outer semiconducting layer consisted of an ethylene butyl acrylate copolymer with an addition of about 40% by weight carbon black.
The testing of the dielectric strength was carried out on these test cables in accordance with a method developed by Alcatel AG & Co, Hannover, Germany, and described in an article by Land H. G., Schadlich Hans, "Model Cable Test for Evaluating the Ageing Behaviour under Water Influence of Compounds for Medium Voltage Cables", Conference Proceedings of Jlcable 91, 24-28 June 1991, Versaille, France. As a value of the dielectric strength is stated 63% of Emax from Weibull diagram in kV/mm. The dielectric strength was measured on the one hand A) after ageing for 16 h at 90° C. in air and, on the other hand, B) after ageing for 1000 h at 9 kV/mm in 85/70° C. water. The results of the testing are stated in the Table below.
______________________________________ Dielectric Strength Composition A (kV/mm) B (kV/mm) ______________________________________ Polymer A (reference) 77.9 39.6 Polymer B (prior art) 95.6 40.6 Polymer C (inventive) 93.6 45.4 ______________________________________
As appears from the test results, the composition according to the invention showed good properties as inner semiconducting layers and, in particular, had an excellent electric dielectric strength after ageing for 1000 h at 9 kV/mm in 85/70° C. water.
In this Example, the dielectric strength was tested in a similar fashion as in Example 3 on an electric cable, which had as inner semiconducting layer a composition consisting of a terpolymer of ethylene, about 15% by weight methylacrylate and about 2% by weight hexaethylene glycol monomethacrylate, i.e. the same monomer of formula I as in Example 3, and about 40% by weight carbon black. In the testing, a dielectric strength (63% of Emax) of 59.4 kV/mm was obtained after ageing for 1000 h at 9 kV/mm in 85/70° C. water.
In this Example, the environmental stress cracking resistance (ESCR) was tested, a property that is important, particularly to the outer semiconducting layer of an electric cable. The testing was carried out according to ASTM D 1693, on the one hand with 10% Igepal at 50° C. and, on the other hand, in air at 50° C.
Three semiconducting polymer compositions (Polymer 1, 2 och 3) were tested, and their compositions were as follows.
Polymer 1 (comparative composition): ethylene vinyl acetate copolymer having 9% by weight vinyl acetate and an MFR=9.5 dg/10 min. Moreover, the composition contained about 36% by weight carbon black.
Polymer 2 (comparative composition): ethylene vinyl acetate copolymer having 18% by weight vinyl acetate and an MFR=9 dg/10 min. Moreover, the composition contained about 40% by weight carbon black.
Polymer 3 (according to the invention): ethylene vinyl acetate hydroxyethyl methacrylate terpolymer having 9% by weight vinyl acetate, 10% by weight hydroxyethyl methacrylate and an MFR=6 dg/10 min. Moreover, the composition contained about 36% by weight carbon black.
The result of the testing of ESCR is shown in the Table below and is stated as the number of test pieces of a total of 10 test pieces that were broken in the testing after a certain time, measured in hours.
______________________________________ Number of broken test pieces/number of hours Composition ESCR, air 50° C. ESCR, 10% Igepal ______________________________________ Polymer 1 10/0 9/0 Polymer 2 3/4 9/1.5 Polymer 3 1/6 7/24 ______________________________________
As appears from the test results, the composition according to the invention had a considerably improved ESCR and is consequently well suited as material for outer semiconducting layers of electric cables.
Claims (16)
1. A composition for electric cables, the composition comprising an ethylene copolymer which includes as a comonomer a (poly)alkylene glycol mono(meth)acrylate having the formula I ##STR2## wherein R1 =H or CH3, n=1-20 and wherein the composition forms a semi-conducting layer of an electric cable and further comprises carbon black in an amount sufficient to make the composition semiconducting.
2. The composition as claimed in claim 1, wherein R1 =CH3, R2 =H, and n=1-10.
3. The composition as claimed in claim 1, wherein n=1.
4. The composition as claimed in claim 1, wherein n=6.
5. The composition as claimed in claim 1, wherein the (poly)alkylene glycol mono(meth)acrylate of formula I constitutes 0.1-15% by weight of the ethylene copolymer.
6. The composition as claimed in claim 1, wherein the ethylene copolymer comprises, in addition to the (poly)alkylene glycol mono(meth)acrylate of formula I, a further vinylically unsaturated comonomer.
7. The composition as claimed in claim 6, wherein the further vinylically unsaturated comonomer is selected from C3 -C8 alpha olefins, (meth)acrylic acid and esters thereof, vinyl acetate, and vinylically unsaturated, hydrolysable silane monomers.
8. The composition as claimed in claim 6, wherein the further vinylically unsaturated comonomer constitutes 1-40% by weight of the ethylene copolymer.
9. A composition for electric cables, the composition comprising an ethylene copolymer which includes as a comonomer a (poly)alkylene glycol mono(meth)acrylate having the formula I ##STR3## wherein R1 =H or CH3, n=1-20 and wherein the composition forms an insulating layer of an electric cable.
10. The composition as claimed in claim 9, wherein R1 =CH3, R2 =H, and n=1-10.
11. The composition as claimed in claim 9, wherein n=1.
12. The composition as claimed in claim 9, wherein n=6.
13. The composition as claimed in claim 9, wherein the (poly)alkylene glycol mono(meth)acrylate of formula I constitutes 0.1-15% by weight of the ethylene copolymer.
14. The composition as claimed in claim 9, wherein the ethylene copolymer comprises, in addition to the (poly)alkylene glycol mono(meth)acrylate of formula I, a further vinylically unsaturated comonomer.
15. The composition as claimed in claim 14, wherein the further vinylically unsaturated comonomer is selected from C3 -C8 alpha olefins, (meth)acrylic acid and esters thereof, vinyl acetate, and vinylically unsaturated, hydrolysable silane monomers.
16. The composition as claimed in claim 9, wherein the further vinylically unsaturated comonomer constitutes 1-40% by weight of the ethylene copolymer.
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PCT/SE1998/000013 WO1998034236A1 (en) | 1997-02-04 | 1998-01-09 | Composition for electric cables |
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